360{20 {0 Azimuth scanning antenna without rotating RF joints

ABSTRACT

A 360* scanning radar antenna has a plurality of primary focusing-structures arranged in a circular fashion illuminating corresponding secondary focusing-structures which in turn are arranged about, and directed toward, a rotating multi-sided halfwave plane reflector. Radar energy is switched to radiate from a given primary focusing-structure during the time when the plane reflector is in position to reflect all of the energy collimated by the corresponding secondary focusing-structure. The secondary focusing-structure may be made to appear transparent to the beam reflected by the plane reflector.

United States Patent OR 3 a 9 16 9 4 .1 6

Lewis Oct. 28, 1975 360 AZIMUTI-I SCANNING ANTENNA WITHOUT ROTATING RF JOINTS Inventor: Bernard L. Lewis, Oxon Hill, Md.

Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

Filed: Sept. 24, 1974 Appl. No: 508,777

U.S. Cl. 343/756; 343/761; 343/779; 343/840; 343/872 Int. C1. HOlQ 19/00; I-101Q 19/12 Field of Search 343/756, 761, 779, 839, 343/840, 872

References Cited UNITED STATES PATENTS Kay 343/756 Primary E.raniinerEli Lieberman Attorney, Agent, or FirmR. S. Sciascia; Arthur L. Branning; Norman V. Brown 57 ABSTRACT A 360 scanning radar antenna has a plurality of primary focusing-structures arranged in a circular fashion illuminating corresponding secondary focusingstructures which in turn are arranged about, and directed toward, a rotating multi-sided half-wave plane reflector. Radar energy is switched to radiate from a given primary focusing-structure during the time when the plane reflector is in position to reflect all of the energy collimated by the corresponding secondary focusing-structure. The secondary focusing-structure may be made to appear transparent to the beam reflected by the plane reflector.

6 Claims, 3 Drawing Figures US; Patent 'Oct.28,1975 Sheet 1 of2 3,916,416

US. Patent Oct.28, 1975 Sheet20f2 3,916,416

360 AZIMUTH SCANNING ANTENNA WITHOUT ROTATING RF JOINTS Background of the Invention The present invention relates to high-frequency scanning radar antenna systems, and more particularly to 360 high-frequency scanning antennas not having rotating RF joints or signal processing on the antenna.

High-frequency, or radar scanning antennas, generally search 360 in azimuth with beams arranged in parallel stacked sets. Each beam searches or scans in its own portion of elevation. Also, each beam of a stacked set may be operated and utilized independently of the other beams, allowing multiple functions to be performed by the antenna system. For example, simultaneous transmission at different frequencies, or searching with one beam while communicating with another may readily be accomplished with multi-function multibeam antennas.

In the past, stacked beam antenna systems have had to resort to separate rotary RF joints for each beam in order to carry the radar signals between the antenna and the signal processing electronics. Unfortunately, realities of antenna construction limit the number of RF joints which may be utilized, and therefore limit the number of beams available utilizing the rotary joint design.

To avoid the necessity of having RF joints, some beam scanning radar systems utilize data processing devices located on the antenna. Processing the radar signal by devices located on the antenna is objectionable because of various factors including increased topside weight and the difficulity in performing maintenance on electronic equipment mounted on the antenna.

Some existing antenna systems utilize single primary and secondary focusing-structures to direct polarized energy onto a moving plane reflector. The plane reflector then changes the direction of polarization and reflects the energy back through the secondary focusingstructure. These systems are only able to search over a limited scanning sector.

The present invention overcomes all of these longstanding limitations and problems by providing a scanning radar antenna operating without the need for RF joints or signal processing on the antenna, and which scans the entire 360 sector.

SUMlVIARY OF THE INVENTION The present invention is a multiple function scanning antenna system which employs a plurality of stationary primary focusing-structures; each of which has an associated polarization-sensitive secondary focusingstructure.

The primary and secondary focusing-structures are arranged in a circular fashion about a rotating multisided half-wave plane reflector. The polarizationsensitive secondary focusing-structure collimates the energy radiated from its corresponding primary focusing-structure and directs it toward the multi-sided halfwave reflecting structure.

High-frequency energy is switched so as to radiate polarized high-frequency energy from a given primary focusing-structure during the time when one of the the secondary focusing-structure. The reflected energy then passes either around the secondary focusingstructure or through it. The secondary focusingstructure appears transparent to the polarization shifted energy.

It is therefore an object of the present invention to scan a high-frequency antenna beam through 360 without resort to rotary RF joints or to signal processing on the antenna.

It is another object of the present invention to generate a parallel stacked set of antenna beams for scanning 360 in azimuth without resort to rotating RF joints or to signal processing on the antenna.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a pictorial view of an embodiment of the antenna system of the present invention.

FIG. 2 depicts in top view the embodiment of FIG. 1.

FIG. 3 indicates the scan angles available for each feed cluster from one side of a half-wave plate reflector with no scan loss, for the embodiment of the invention depicted in FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION Although the present invention is useful for all highfrequency electromagnetic systems, this description will refer only to radar frequencies.

Referring now to FIG. 1, an antenna system enclosure 10 is constructed of material which is strong and environmentally stable, such as fiberglass, and which is essentially transparent to electromagnetic energy. Enclosure 10 is supported above an antenna support platform 12 by support members 14. Enclosure 10 is generally circularly shaped when viewed from the top and generally elliptical in shape when viewed from the side.

In the center of enclosure 10 is a tertiary reflector in the form of a two-sided plane reflector 16. Plane reflector 16 is constructed as a half-wave plate. Reflector 16 is mounted on a reflector rotating assembly 18, itself mounted upon a pedestal 20 which passes through enclosure 10 and is attached to support platform 12.

symmetrically arranged around pedestal 20 are a set of four primary focusingstructures or feed-clusters 22. Each feed-cluster 22 is formed by a rectangular array of individual radiating elements 24. Each radiating element 24 is fed radar energy through a switching network (not shown) from a source of radar energy (not shown). Also, each radiating element 24 is designed to independently radiate energy polarized in the vertical direction (E field vertical).

In front of each feed-cluster 22 is a corresponding polarization-sensitive secondary focusing-structure, or parabolic reflector 26. Plane reflector 16 is constructed to have a width of l/cos 45 1.414 times the width of the parabola. Each polarization-sensitive reflector 26 is constructed so as to reflect vertically polarized energy while passing horizontally polarized energy unhindered.

Each parabolic reflector 26 is mounted immediately adjacent to the inner surface of enclosure 10, and disposed so that the optical axis of each passes through the same point located on the axis of rotation of half-wave reflector 16. The center of each feed-cluster 22 is located at a point on the focal surface but below the focal point of its corresponding parabolic reflector 26.

In operation, vertically polarized radar-frequency energy is radiated from a radiating elements 24 of feedclusters 22, and directed at corresponding polarizationsensitive parabolic reflectors 26. Each polarizationsensitive reflector 26 allows horizontally polarized energy to pass unhindered while reflecting vertically polarized energy. (Construction and operation of reflectors of this nature are well-known in the antenna field and will not be further discussed.) Reflectors of this nature are discussed beginning at page 298 of Antenna Analysis by Edward A. Walfi', John Willey & Sons, 1966. The vertically polarized electromagnetic radiation from a radiating element 24 will be reflected and collimated by its corresponding parabolic reflector 26. Since each feed-cluster 22 is located on the focal surface but below the focal point of the secondary focusing-structure 26, the collimated-reflected beam will generally be reflected at an angle with the optical axis equal to the angle between the radiating element and the optical axis.

The collimated beam then impinges upon a half-wave plane reflector surface 17 of multi-sided reflector 16, which changes the direction of polarization of the beam by 90 and reflects it back (with the angle of reflection equal to the angle of incidence).

The beam from a particular radiating element 25, located close to the optical axis of its corresponding parabolic reflector 26, leaves the parabolic reflector 26 at a small angle with respect to the optical axis. This beam is thus reflected by plane reflector 17 back through its corresponding parabolic reflector 26. The beam from a particular radiating element 27, located further from the optical axis, leaves the corresponding parabolic reflector at a greater angle so as to pass over parabolic reflector 26.

Turning now to FIG. 2, the two half-wave reflective sides 17 of reflector 16 are denoted as X and Y. Reflector 16 is continuously rotated in a clockwise manner, indicated by arrow 19, by rotating means 18 (shown in FIG. 1). Feed-clusters 22, arranged in circular manner about the axis of rotation of reflector 16, are denoted as A, B, C, and D. Their corresponding parabolic reflectors 26 are denoted as A, B, C, and D.

Selected feed-clusters 22, such as cluster A, will be switched on to radiate its energy when one of the plane reflecting surfaces 17 (e.g. X) is positioned so that the energy from the selected feed-cluster, as focused and collimated by its corresponding parabolic reflector (e.g. A), will be reflected by the plane reflector surface 17.

In the present embodiment, since the width of each reflective surface 17 is 1/cos 45 (or 1.414) times the width in the scan plane of the parabolic reflectors 26, all collimated energy from a particular parabolic reflector 26, will be reflected when the plane reflector surface 17 is at an angle of no more than 45 with respect to the optical axis of the parabolic reflector 26.

When at its extreme angular position (i.e., 45) with respect to the optical axis of the particular parabolic reflector 26, plane reflector-surface 17 will reflect incident energy at an angle of 90. As the plane reflectorsurface moves from 45 to one side to 45 on the other side of the optical axis, energy from the associated feedcluster will be focused and collimated by the corresponding parabolic reflector. The resulting beam will then be swept from 90 on one side of the optical axis of the parabolic reflector to 90 on the other. Thus it is clear that a particular feed-cluster, utilized in conjunction with its corresponding parabolic reflector (forming a focusing pair A-A') cover a scan-width of 1 In a similar manner each of the other focusing pairs B-B', C-C' and D-D' also cover 180. When utilized in the symmetrical arrangement of the present embodiment, scans of adjacent pairs overlap, as illustrated in FIG. 3. FIG. 3 shows the scan coverage for the embodiment of the present invention depicted in FIGS. 1 and 2. It is clear from FIGS. 2 and 3 that opposite focusing pairs cooperate to scan 360 twice per revolution of plane reflector 16.

If back-to-back scanning in the manner described above, using two or more plane reflecting surfaces 17 of a multi-sided reflector is not desired, another embodiment of the present invention will be more appropriate. In this alternative embodiment, each focusingpair (of feed cluster 22 and parabolic reflector 26) can be used to scan and the length of the rotating plane reflector 16 can be reduced from 1/cos 45 (or 1.414) to 1 /cos 22.5 (or 1.082) times the parabola length in the scan plane. However, this will result in dead times (between 90 scans) equal to the time required for the rotating reflector to move 45. In many high-frequency systems, a certain amount of dead time is desirable to permit switching of energy between the various feed clusters, and the dead time is then a useful advantage.

Other embodiments of the present invention may utilize a multi-sided plane reflector having a plurality n of plane reflector surfaces used in conjunction with an even numbered plurality m of parabolic reflectors and feed horns. In this instance the number of 360 scans per revolution is equal to n.m/2. For example, a foursided reflector used in conjunction with four parabolic reflectors produces 4.4/2 8 scans per revolution.

Still another embodiment of the present invention utilizes 8 focusing pairs formed by 8 feed clusters 22 in conjunction with 8 corresponding parabolic reflectors 26 and a smaller double-sided rotating plane reflector 16. This arrangement would produce continuous backto-back scans with each focusing pair covering 45. An arrangement of this nature would be particularly useful for a rapid-scanning high-frequency radar for use against pop-up targets.

The preferred embodiment of the present invention utilizes polarized energy in conjunction with a polarization altering half-wave reflecting surface 17, and polarization sensitive parabolic reflector 26. Although utilization of these polarization sensitive devices permit properly polarized energy to pass through the polarization sensitive parabolic reflector, the ability for the beam to pass back through the parabolic reflector may not be needed in all situations. For example, if the antenna beams were scanning only the region above the parabolic reflectors 26, the polarization-sensitive capability would be unnecessary.

Although the embodiments of the present invention herein described assume a generally horizontal orientation of antenna arrangement, other orientations may be utilized. Also, it may be advantageous to arrange focusing-pairs in a non-symmetric manner about the rotating tertiary plane reflector 17. A non-symmetric arrangement may be employed, for example, to produce a higher repetition rate in one (or several) direction than in other directions.

As has been described above, the present invention enables scanning in 360 of a stacked set of highfrequency beams. It provides also for scanning emphasis in one or more selected directions while incorporating the additional capability of transmitting and receiving properly polarized beams through the secondary focusing-structures.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise then as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A 360 scanning high-frequency antenna system comprising:

an antenna platform;

a supporting framework surrounding and attached to said platform;

tertiary reflecting means for reflecting electromagnetic energy;

rotating means attached between said tertiary reflecting means and said platform for rotating said tertiary reflecting means about an axis of rotation;

a plurality of high-frequency primary focusingstructures attached to said supporting framework and arranged in a generally circular fashion about said axis of rotation;

a plurality of secondary focusing-structures attached to said supporting framework and arranged in a generally circular fashion about said axis of rotation, each said secondary focusing-structure being associated with a corresponding one or said primary focusing-structures; and,

switching means for energizing selected primaryfocusing structures when said tertiary reflecting means is positioned to reflect energy radiating from said selected primary focusing-structures as collimated by said corresponding secondary focusingstructure whereby said energy is reflected by said tertiary reflecting means.

2. The 360 scanning antenna system of claim 1, wherein:

said primary focusing-structures are adapted to radiate linearly polarized energy of predetermined orientation;

said secondary focusing-structures are made polarization sensitive so as to reflect energy polarized in one direction and to pass orthogonally polarized energy without attenuation;

said tertiary reflecting means is a reflector having a plurality of half-wave plane reflective surfaces constructed so that the direction of polarization energy incident on any of said half-wave reflective surfaces will be changed by upon reflection, whereby energy from said primary focusingstructures polarized in one direction is collimated and reflected to said tertiary reflecting means by said secondary focusing structures and said reflected energy is rotated in polarization by 90 and reflected back through said secondary focusingstructures by said tertiary reflecting means.

3. The 360 scanning antenna system of claim 2 wherein the width of said tertiary reflecting means is l/cos 45 times the width in the scan plane of one of said secondary focusing-structures.

4. The antenna system of claim 3 wherein said switching means energizes selected primary focusingstructures when said tertiary reflector forms an angle of not more than 45 to either side of the optical axis of said corresponding secondary focusing-structure.

5. The 360 scanning antenna system of claim 4 wherein a environment stable, electromagnetically transparent enclosure is attached to and encloses said supporting framework.

6. The 360 scanning antenna system of claim 5 wherein said enclosure is constructed of fiberglass. 

1. A 360* scanning high-frequency antenna system comprising: an antenna platform; a supporting framework surrounding and attached to said platform; tertiary reflecting means for reflecting electromagnetic energy; rotating means attached between said tertiary reflecting means and said platform for rotating said tertiary reflecting means about an axis of rotation; a plurality of high-frequency primary focusing-structures attached to said supporting framework and arranged in a generally circular fashion about said axis of rotation; a plurality of secondary focusing-structures attached to said supporting framework and arranged in a generally circular fashion about said axis of rotation, each said secondary focusing-structure being associated with a corresponding one or said primary focusing-structures; and, switching means for energizing selected primary-focusing structures when said tertiary reflecting means is positioned to reflect energy radiating from said selected primary focusingstructures as collimated by said corresponding secondary focusing-structure whereby said energy is reflected by said tertiary reflecting means.
 2. The 360* scanning antenna system of claim 1, wherein: said primary focusing-structures are adapted to radiate linearly polarized energy of predetermined orientation; said secondary focusing-structures are made polarization sensitive so as to reflect energy polarized in one direction and to pass orthogonally polarized energy without attenuation; said tertiary reflecting means is a reflector having a plurality of half-wave plane reflective surfaces constructed so that the direction of polarization energy incident on any of said half-wave reflective surfaces will be changed by 90* upon reflection, whereby energy from said primary focusing-structures polarized in one direction is collimated and reflected to said tertiary reflecting means by said secondary focusing structures and said reflected energy is rotated in polarization by 90* and reflected back through said secondary focusing-structures by said tertiary reflecting means.
 3. The 360* scanning antenna system of claim 2 wherein the width of said tertiary reflecting means is 1/cos 45* times the width in the scan plane of one of said secondary focusing-structures.
 4. The antenna system of claim 3 wherein said switching means energizes selected primary focusing-structures when said tertiary reflector forms an angle of not more than 45* to either side of the optical axis of said corresponding secondary focusing-structure.
 5. The 360* scanning antenna system of claim 4 wherein a environment stable, electromagnetically transparent enclosure is attached to and encloses said supporting framework.
 6. The 360* scanning antenna system of claim 5 wherein said enclosure is constructed of fiberglass. 